Rare earth oxysulfide ceramics

ABSTRACT

Rare earth oxysulfide ceramic material prepared by covering the material with a foil of a metal selected from the group consisting of molybdenum, tungsten, platinum, and rhenium, sealing it into an airtight vessel of tantalum or niobium, and then subjecting it to a hot isostatic press process.

This is a division of application Ser. No. 004,574, filed Jan. 20, 1987U.S. Pat. No. 4,752,424.

BACKGROUND OF THE INVENTION

The present invention relates to a method of manufacturing a ceramicand, more particularly, to a method of manufacturing a rare earthoxysulfide ceramic.

According to a description of Japanese Patent Disclosure (Kokai) No.58-204088, a ceramic of a fluorescent material obtained by partiallysubstituting a rare earth oxysulfide (RE₂ O₂ S; RE is a rare earthelement) with another rare earth element such as Gd₂ O₂ S:Pr can be usedfor a scintillation detector. In this case, in order to obtain a largeamount or light emission from the ceramic, the ceramic must have smalllight loss and high light transmittance. In addition, coloration, i.e.,light absorptin of the ceramic, and light scattering due to pores orsegregates inside the ceramic must be suppressed.

A ceramic with less pores and less inclusions is conventionallymanufactured by the hot press method or the hot isostatic press method.This is because an additive material may be left as segregates in thepressureless-sintering process when it is used to obtain high-densityceramics.

However, in the hot press method; no shield is present between a ceramicmaterial and ambient atmosphere and the ceramic material tends to beadversely affected by high-temperature atmosphere. Therefore, when arare earth oxysulfide likely to be decomposed at a high temperature isused as a material, coloration or degradation of emission efficiency ofthe ceramic occurs upon decomposition.

On the other hand, with the hot isostatic press method in which amaterial is sealed in an airtight vessel and a hot isostatic pressprocess is performed, the above decomposition does not occur because ashield is present between the material and the atmosphere. However,since the airtight vessel and the ceramic material are in direct contactwith each other, coloration tends to occur due to the reaction betweenthe vessel and the material or diffusion of a metal constituting thevessel into the ceramic.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of the aboveproblems, and has as its object to provide a method of manufacturing aceramic capable of suppressing coloration produced in the hot isostaticpress.

The present invention is characterized by covering a ceramic materialwith a foil of a metal selected from the group consisting of molybdenum,tungsten, platinum, and rhenium, sealing the resultant material into anairtight vessel made of tantalum or niobium, and performing the hotisostatic press.

An example of the above ceramic material is a rare earth oxysulfide suchas a gadolinium oxysulfide, or a gadolinium oxysulfide obtained bypartially substituting gadolinium with another rare earth element. Sucha ceramic material may be subjected to the hot isostatic press processin a powdery state, but is preferably cold-pressed in advance to obtaina high density for easy handling and sintering.

An airtight vessel used for the above hot isostatic press process mustsatisfy the following three conditions: (1) since the hot isostaticpress is performed at a high temperature, the vessel must be made of ahigh melting-point material capable of maintaining functions as anairtight vessel even at high temperatures; (2) the vessel must easilyundergo plastic deformation because the vessel must be able to transferpressure to the ceramic material during the hot isostatic press and, atthe same time, the vessel itself must collapse as the ceramic materialshrinks upon sintering; and (3) formation of the vessel and airtightsealing must be easily obtained. Tantalum and niobium satisfy the aboveconditions, and especially tantalum is effective. On the contrary,tungsten, for example, is not suitable due to poor plastic deformationand molding properties, although it is a refractory metal and satisfiesthe above condition in item (1).

A foil used in the above hot isostatic press process serves to preventthe ceramic material from directly contacting the airtight vessel duringthe press and must be made of molybdenum, tungsten, platinum, orrhenium, as described above. For example, although tantalum is suitablefor the airtight vessel for the above hot isostatic press process, itdegrades light transmittance of a rare earth oxysulfide due tocoloration produced during the hot isostatic press when it is broughtinto contact with the rare earth oxysulfide as the ceramic material. Bythe way, if the foil is too thick, it cannot sufficiently transfer thepressure to the ceramic material because of its strength. Therefore, thethickness of the foil made by conventional methods is desirably lessthan 200 μm.

When the hot isostatic press temperature is low, sintering of the rareearth oxysulfide does not progress smoothly. At temperatures of about1,300° C. or less, more pores are formed in the ceramic and lighttransmittance is degraded. On the other hand, at high temperaturesexceeding about 1,800° C., coloration is produced due to decompositionof the rare earth oxysulfide or reaction thereof with the foil even whenthe above foil is used, so that light transmittance and luminousefficiency are degraded. Therefore, the hot isostatic press temperatureis preferably between about 1,300° and 1,800° C. and, more preferably,between 1,450° to 1,650° C.

In the above hot isostatic press process, if the pressure is too low,the rare earth oxysulfide is insufficiently sintered and the number ofpores formed in the ceramic increases to degrade light transmittance.For this reason, a lower limit of pressure is preferably a severalhundreds atm. (several tens of MPa).

As for the timings of pressurization and heating in the hot isostaticpress process, pressurization may be performed first and then heatingmay be performed. However, the airtight vessel may be destroyed duringthe process according to this method. Therefore, it is preferred toperform heating first, up to a temperature around 1,000° C., so that theairtight vessel is softened, and then to perform pressurization at apredetermined pressure. Thereafter, heating is performed again at apredetermined temperature. This method is more preferable in terms ofyield because the airtight vessel tends to be destroyed less often.

As described above, a ceramic material is not directly sealed into anairtight vessel made of tantalum or niobium. It is covered in advancewith a foil of a metal selected from the group consisting of molybdenum,tungsten, platinum, and rhenium and then sealed into the airtight vesselso as not to contact the vessel. Thereafter, the hot isostatic press ofthe material is performed to obtain a translucent rare earth oxysulfideceramic with less pores and coloration.

Note that in the present invention, in order to reduce coloration of aceramic (especially, a rare earth oxysulfide ceramic), a heat treatmentof a rare earth oxysulfide may be effectively performed in an open airatmosphere prior to the hot isostatic press process. In the heattreatment in an open air atmosphere, the surface of a rare earthoxysulfide (RE₂ O₂ S) powder particles is oxidized to produce a smallamount of RE₂ O₂ SO₄, thereby suppressing coloration. The temperature ofthe heat treatment is preferably 400° to 800° C. so that only RE₂ O₂ Son the surface is converted into RE₂ O₂ SO₄ but RE₂ O₂ S inside remainsnon-oxidized. Although it changes depending on the treatmenttemperature, a heat treatment time is preferably about 30 min. to 3hours. The heat treatment may be performed before or after cold-pressingof the material powder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a package of Gd₂ O₂ S:Pr, a molybdenumfoil, and an airtight vessel of tantalum in a hot isostatic pressprocess of a Gd₂ O₂ S:Pr ceramic according to Example 1 of the presentinvention;

FIG. 2 is a schematic view showing a method of measuring diffusetransmittance; and

FIG. 3 is a schematic view showing behaviors of light produced when anexcitation beam such as an ultraviolet beam, an electron beam, an X-raybeam, or a γ beam becomes incident on a fluorescent ceramic.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1

A powder of a gadolinium oxysulfide fluorescent material activated bypraseodymium (Gd₂ O₂ S:Pr) was subjected to a cold isostatic press undera pressure of about 200 MPa and then shaped. A resultant material, i.e.,cold-pressed fluorescent material 1, was covered with molybdenum foil 2having a thickness of 40 μm. Subsequently, the resultant material wascharged into a cylindrical capsule of tantalum having a thickness of 0.3mm, an inner diameter of 40 mm, and a height of 50 mm. After theinternal air was exhausted, airtight vessel 3 was completed by electronbeam welding (FIG. 1). Thereafter, the airtight vessel was pressurizedup to 50 MPa at room temperature using argon as a pressure medium,heated under pressure, and then maintained under the final conditions of1,700° C. and 150 MPa for 1 hour. A translucent ceramic of a Gd₂ O₂ S:Prfluorescent material was obtained by this hot isostatic press process.

The bulk density of the above Gd₂ O₂ S:Pr ceramic was 100% with respectto crystallographic density of Gd₂ O₂ S:Pr. That is, the volumepercentage of pores in the ceramic was 0. The ceramic was light gray incolor, but exhibited almost no trace of coloration. The ceramic was cutinto a sample plate having a thickness of 1 to measure diffusetransmittance with respect to an He--Ne laser beam, regulartransmittance with respect to white light when slits were arrangedbefore and behind a sample, and diffuse reflectance when the sample wasplaced on a white paper. As a result, diffuse transmittance was 23%,regular transmittance was 20%, and diffuse reflectance was 38%. In orderto measure diffuse transmittance, He--Ne laser beam 5 was incident ontoceramic sample 4, and transmitted light 10 entered integrating sphere 9,having a diameter of 150 mm, and was then measured by detector 11, asshown in FIG. 2. The ceramic sample was a square plate, 9 mm×9 mm, witha certain thickness, e.g. 1 mm. The square surface of the plate servingas a light incident/reflecting surface was polished to obtain a mirrorsurface, and its sides were coated by a white paint to prevent leakageof light. An opening through which the transmitted light entered theintegrating sphere was a square opening having the same size as that ofthe ceramic sample, and the sample was placed at the opening. Diffusereflectance was measured by placing a sample plate with a predeterminedthickness and two mirror surfaces on white paper and using a Macbethoptical densitometer (RD918). In addition, Sample 4 of the ceramic wascut out into a plate having a thickness of 2 mm, a γ beam as incidentexcitation beam 12 was radiated from a ⁵⁷ Co beam source (not shown)onto sample 4, as shown in FIG. 3, and light emission 6 was observed. Asa result, although light emission toward the excitation beam 7 wasfound, luminous intensity of light emission observed from a transmitting(output) side 8 was sufficiently strong.

Example 2

A powder of Gd₂ O₂ S:Pr was put into an alumina crucible and subjectedto a heat treatment in an electric furnace in an open air atmosphere ata temperature of 600° C. for 3 hours. Thereafter, following the sameprocedures as in Example 1, a translucent ceramic of Gd₂ O₂ S:Pr wasmanufactured.

The bulk density of the resultant Gd₂ O₂ S:Pr ceramic was 100%. Theresultant Gd₂ O₂ S:Pr ceramic had a white color except for a surfaceportion which was in direct contact with a molybdenum foil, andexhibited no coloration. The diffuse transmittance, regulartransmittance and diffuse reflectance of the ceramic were measured inthe same manner as in Example 1 and found to be 39%, 30% and 49%,respectively.

Example 3

Following the same procedures as in Example 1, a cold-pressed Gd₂ O₂S:Pr was covered with a molybdenum foil and then charged into a tantalumcapsule to complete an airtight vessel. Subsequently, the vessel washeated to 1,100° C. and pressurized to about 75 MPa using argon as apressure medium while it was kept at the same temperature. Thereafter,the vessel was subjected to heating under pressure again and maintainedunder the final condition of 1,500° C. and 90 MPa for 3 hours. By thishot isostatic press process, a translucent ceramic of Gd₂ O₂ S:Pr wasobtained.

The bulk density of the resultant Gd₂ O₂ S:Pr ceramic was 99.9%, and theceramic was light gray in color. Diffuse transmittance of the ceramicmeasured in the same manner as in Example 1 was 28.5%, luminousintensity observed at the transmitting side during irradiation of a γbeam was 190% compared with that of Example 1.

Example 4

Following the same procedures as in Example 1, except that a platinumfoil was used instead of a molybdenum foil and the temperature andpressure during the hot isostatic press process were limited to 1,600°C. and 1,400 atm. (140 MPa) respectively, a translucent ceramic of Gd₂O₂ S:Pr was manufactured.

The bulk density of the resultant Gd₂ O₂ S:Pr ceramic was 100%, andthere was little internal coloration. The diffuse transmittance, diffusereflectance and regular transmittance of the ceramic measured in thesame manner as in Example 1 were 49%, 42% and 32%, respectively.

Example 5

Following the same procedures as in Example 3, except that a capsule ofniobium was used instead of that of tantalum, a translucent ceramic ofGd₂ O₂ S:Pr was manufactured.

The bulk density of the resultant Gd₂ O₂ S:Pr ceramic was 99.9%. Thediffuse reflectance and diffuse transmittance of the ceramic measured inthe same manner as in Example 1 were 28% and 30%. The luminous intensityobserved at the transmitting side during irradiation of a Y beam wassubstantially the same as in Example 3.

Example 6

A Gd₂ O₂ S:Pr fluorescent material powder was subjected to the coldisostatic press under a pressure of about 200 MPa and then shaped. Theresultant material was subjected to a heat treatment in an electricfurnace in an open air atmosphere at a temperature of 600° C. for 1hour. Subsequently, the resultant material was covered with a molybdenumfoil and charged into a tantalum capsule, thereby completing an airtightvessel. Thereafter, the vessel was subjected to the hot isostatic pressin the same manner as in Example 1, so that a translucent ceramic of Gd₂O₂ S:Pr was manufactured.

The bulk density of the resultant Gd₂ O₂ S:Pr was 100%. The resultantGd₂ O₂ S:Pr ceramic was light gray in color,except for a surface whichwas in direct contact with the molybdenum foil. The regulartransmittance, diffuse transmittance and diffuse reflectance of thereflectance of the ceramic, measured in the same manner as in Example 1,were 23%, 26% and 38%, respectively.

Control 1

Using a carbon mold coated with boron nitride powder as an inner-liningmaterial, a Gd₂ O₂ S:Pr ceramic was manufactured by a hot press processin vacuum under the conditions of 1,600° C. and 40 MPa.

The bulk density of the resultant ceramic was 99.6%, and the ceramic wasgray in color. The diffuse transmittance of the ceramic with respect toan He--Ne laser beam, measured in the same manner as in Example 1, was1% or less, and light emission observed at the transmitting side duringirradiation of a γ beam was less than a detection limit. Thus, a rareearth oxysulfide ceramic manufactured by a method other than the hotisostatic press using an airtight vessel has significantly degradedlight transmittance and luminous efficiency.

Example 7

A Gd₂ O₂ S:Pr powder was cold-pressed, shaped, covered with a molybdenumfoil, and then sealed into an airtight vessel of tantalum. The resultantsample was heated to 1,100° C. in an argon atmosphere and thenpressurized up to about 75 MPa using argon as a pressure medium while itwas kept at the same temperature. The sample was again subjected toheating under pressure again and maintained under the final conditionsof 1,450° C. and 90 MPa for 3 hours. By this hot isostatic pressprocess, a Gd₂ O₂ S:Pr ceramic was obtained. The bulk density of theceramic was 99.9% with respect to crystallographic density, and thediffuse transmittance of a 2 mm-thick sample with respect to light inthe visible range was 30.5%.

When X-rays were radiated onto a detector obtained by bringing a siliconphotodiode into contact with a 2 mm-thick ceramic, a signal indicated ahigh sensitivity 220% that of a detector obtained by combining a CdWO₄single crystal and a silicon photodiode. In addition, using ²⁴¹ Am and⁵⁷ Co as γ beam sources having different photon energies (60 keV and 122keV, respectively), a ratio of signals of the ceramic detector and theCdWO₄ single-crystal detector described above was examined forrespective γ beams. A ratio of the magnitudes of the signals for the γbeam from ⁵⁷ Co was larger by 1.5% than that from ²⁴¹ Am. This indicatesthat linearity of the detector using a ceramic in Example 7 with respectto energies of 60 to 122 keV is degraded only by 1.5% that of the CdW0₄single-crystal detector.

Example 8

Following the same procedures as in Example 7, except that the finaltreatment temperature was 1,600° C. and the final treatment pressure was97 MPa, a Gd₂ O₂ S:Pr ceramic was manufactured. The bulk density of theceramic was 100% with respect to true specific gravity, and the diffusetransmittance of a 2mm-thick sample with respect to light in visiblerange was 20.5%. An output signal of a detector obtained by combiningthe above 2 mm-thick ceramic and a silicon photodiode was 160% that of aCdW0₄ single-crystal detector. The linearity, measured in the samemanner as in Example 7, was lower only by 8.5% than that of the CdW0₄single-crystal detector. That is, an amount of deviation was found to besmall.

Example 9

Following the same procedures as in Example 7, except that lanthanumoxysulfide obtained by partially substituting lanthanum with terbium(La₂ O₂ S:Tb) was used as a material, and that the final treatmenttemperature was 1,500° C. and the final treatment pressure was 90 MPa,the hot isostatic press process was performed to manufacture an La₂ O₂S:Tb ceramic. The bulk density of the ceramic was 99.9% with respect tocrystallographic density, and the diffuse transmittance of a 2 mm-thicksample with respect to light in visible range was 30.0%. An outputsignal of a detector obtained by combining the above 2 mm-thick ceramicand a silicon photodiode was 330% that of a CdW0₄ single-crystaldetector. In addition, linearity measured in the same manner as inExample 7 was degraded only by 2.2% than that of the CdW0₄single-crystal detector. That is, an amount of deviation was found to besmall.

Control 2

A Gd₂ O₂ S:Pr powder was cold-pressed under a pressure of 200 MPa, andthen cut into a 2 mm-thick sample. The transmittance of this pressedmaterial was 0. This material was combined with a photodiode to obtain adetector. But an output signal of the detector during irradiation of anX-ray beam was under the detection limit.

Control 3

A Gd₂ O₂ S:Pr powder was cold-pressed, shaped and then directly sealedinto an airtight vessel of tantalum without using a molybdenum foil.Thereafter, the vessel was pressurized to 50 MPa at room temperatureusing argon as a pressure medium. Then, the vessel was subjected toheating under pressure and maintained under the final conditions of1,700° C. and 150 MPa for 1 hour. With this hot isostatic press process,a Gd₂ O₂ S:Pr ceramic was manufactured. The bulk density of theresultant Gd₂ O₂ S:Pr ceramic was 100% with respect to crystallographicdensity, as in the case with Example 1. However, this ceramic was purplegray in color and exhibited severer coloration than that of the ceramicin Example 1. A plate having a thickness of 1 mm was cut from theceramic, as in the case with Example 1, and diffuse transmittance withrespect to an He--Ne laser beam was measured. As a result, diffusetransmittance was 16%, which was lower than that of the ceramic inExample 1. In addition, the ceramic was cut into a plate having athickness of 2 mm, and a γ beam was radiated onto this sample. As aresult, luminous intensity observed at the transmitting side was about20% that of Example 1. An output signal of a detector obtained bycombining the above 2 mm-thick ceramic with a silicon photodiode wasabout 20% that of a CdWO₄ single-crystal detector. Furthermore, thelinearity, measured in the same manner as in Example 7, was degraded by17.5% than that of the CdWO₄ single-crystal detector. That is, the aboveceramic was found not to be suitable for practical use.

Note that in the above embodiment, the description has been made withreference to Gd₂ O₂ S:Pr or La₂ O₂ S:Tb. However, the same effect can beachieved by another rare earth oxysulfide or a fluorescent material suchas Y₂ O₂ S:Eu or (La, Gd)₂ O₂ S:Tb.

What is claimed is:
 1. Rare earth oxysulfide ceramics of reducedcoloration and improved emission efficiency manufactured by covering atleast one rare earth oxysulfide ceramic material preptreated by a coldpress with a foil of a metal selected form the group consisting ofmolybdenum, tungsten, platinum, and rhenium, sealing the metalfoil-covered material into an aritight vessel of tantalum or niobiumand, then, subjecting the sealed material to a hot isostatic pressprocess.
 2. A rare earth oxysulfide ceramic according to claim 1,wherein the bulk density is more than 99.9%, and the diffusetransmittance is more than 20.5% when the thickness of the rare earthoxysulfide ceramic is 2 mm.
 3. A ceramic according to claim 1, whereinsaid ceramic is a fluorescent material of a rare earth oxysulfide.